Underbed thread trimming device for sewing machines

ABSTRACT

A device for severing both the bobbin and needle threads of a lockstitch sewing machine in which a movable cutting blade is carried beneath the sewing machine throat plate for cooperation with a ledger blade formed on the bottom surface of a throat plate insert having a needle aperture formed therein.

United- States Patent [191 Kroll '[111 3,783,813 1 Jan. 8, 11974UNDERBED THREAD TRIMMING DEVICE FOR SEWING MACHINES Inventor: GunterKroll, Munich, Germany The Singer Company, New York, NY.

Filed: Aug. 14, 1972 Appl. N0.: 280,701

Assignee:

US. Cl. 112/252 Int. Cl D05b 65/00 Field of Search 112/65, 70, 122,

References Cited 1 1 UNITED STATES PATENTS 2,376,417 5/1945 Chudner112/252 2,826,160 3/1958 Adams 112/252 2,358,878 9/1944 Pikul 112/2522,707,927 5/1955 Artzt et a] 1 12/252 3,084,647 4/1963 Matchett et al.112/252 3,118,405 1/1964 Schenkengel 112/252 3,391,665 7/1968 Kozuka112/252 Primary Examiner--Warner H. Schroeder Attorney- Marshall J.Breen, Chester A. Williams J r.

and Robert E. Smith 1 Claim, 6 Drawing Figures sfiesma PATENTED JAN81974 UNDERBED THREAD TRIMMING DEVICE FOR SEWING MACHINES BACKGROUND OFTHE INVENTION In lockstitch sewing machines and particularly thoseorganized for cyclic sewing to produce predetermined groups ofstitchesas for attaching buttons or sewing tacks, and the like, knownthread trimming devices have the shortcoming that they leaveobjectionably long ends of the severed thread hanging from the work,which shortcoming cannot be overcome by moving the known thread trimminginstrumentalities closer to the stitching point because this wouldinterfere with thread manipulation during stitch formation.

BRIEF SUMMARY'GF THE INVENTION The present invention provides a threadtrimming mechanism for simultaneously severing the bobbin thread and onelimb of the thread loop in a lockstitch sewing machine which mechanismis capable of severing threads extremely close to the stitches which areset in the work. The object of this invention is attained by tron.

DESCRIPTION OF THE DRAWINGS In the accompanying drawings of a preferredembodiment:

FIG. 1 represents a side elevational view of a portion of a sewingmachine havingthe thread trimming device of this invention appliedthereto,

FIG. 2 is 'a bottom plan view of the sewing machine throat plate withthe thread trimming device of this invention illustrated in solid linesin an at-rest position and in dotted lines in a position for picking upthe bob bin thread of the sewing machine,

FIG. 3 is a bottom plan view of the sewing machine throat plate similarto' FlG...2 and showing the thread trimming mechanism of this inventionin solid lines in a position for picking up the needle thread loop andin dotted lines in the final thread severing position,

FIG. 4 is a top plan view of the movable knife blade assembled onto thethread manipulating plate,

FIG. 5 is an enlarged perspective view of the throat plate insert formedwith the ledger blade and of the movable thread severing knife of thisinvention, and

FIG. 6 is a perspective view of a sewing machine loop taker, the thread,manipulating plate, and the throat plate insert showing the arrangementof threads prior to trimming. I

FIG. 1 illustrates a conventional lockstitch cyclic sewing machine ofthe type adapted to produce repetitive stitched tacks such as may beused to sew buttons, snaps or the like to garments. The sewing'machineincludes a work supporting bed 11 in which a loop taker 12 is supported.A throat plate 13 is secured on the work supporting bed to sustain workfabrics against the thrust of a thread carrying needle 14 which isendwise reciprocable in a bracket arm 15 which forms part of the sewingmachine frame and is rigidly supported relatively to the sewing machinebed 11. A work clamp 16 is carried by the sewing machine frame and isshiftably supported between the bed and the bracket arm to carry workfabrics in a predetermined path relatively to the stitching point asdetermined by the needle and the loop taker.

Although not illustrated in the accompanying drawings, group stitchsewing machines such as that illustrated in FIG. 1 conventionallyinclude cam devices for shifting the work clamp, stopping the machineafter completion of a predetermined number of stitches, or operatingancillary devices. It is also conventional in this type of group stitchsewing machine to provide operator influenced mechanisms for effectingvarious operations incidental to the performance of the machine. Any ofthe known conventional cam devices as well as a manually operable devicemay be utilized to shift a thread trimmer operating rod 20 which iscarried beneath the sewing machine throat plate 13 as shown in FIGS. 1and 3. The operating rod is formed with a rack 21 constrained in meshwith a gear segment 22 journaled on a stud shaft 23 supported in thethroat plate and in a bracket 24 secured by a fastening screw 25 beneaththe throat plate. The gear segment which is thus rotatable on a verticalaxis beneath the throat plate on the stud, is secured to rotate with aknife supporting arm 26 which extends radially outward from the. studshaft 23 and is formed with a downturned positioning flange 27 at thefree extremity and with a threaded boss 28. A thread manipulating plate30 is formed with an aperture 31 which embraces the boss 28 and a slot32 which embraces the positioning flange 27 so as to orient the threadmanipulating plate with respect to the supporting arm 26. A securingscrew 33 threaded in the threaded hole 28 clamps the thread manipulatingplate 30 securely in place on the supporting arm. The threadmanipulating plate overhangs a circular recess 34 formed in theunderside of the throat plate 13 and as shown in FIGS. 2 and 3 is formedwith a pair of converging tapered thread deflecting fingers 35 and 36which terminate beneath the circular recess 34. A teardrop shaped slot37 is formed between the tapered converging fingers 35 and 36 which slot37 is located directly beneath the circular recess 34.

Extending radially forwardly and outward from the thread deflectingfinger 36 is an arched thread deflecting finger 38 which terminates in aradially inwardly bent extremity 39 extending transversely across thecircular recess 34 beneath the throat plate.

Secured by fastening screws 40 beneath the thread manipulating plate 30is a thread severing blade 41 formed with a sharpened cutting edge 42which extends substantially radially with respect to the stud shaft 23about which the knife blade moves. The blade 41 is offset from thethread manipulating plate on which it is secured so that the cuttingedge extends into close proximity to the throat plate in the circularrecess 34 therein. The blade 41, further, is formed with a finger 43which is upturned and projects within a guard aperture 44 in the threadmanipulating plate and serves to prevent threads from slipping sidewaysout of the path of the cutting edge 42.

The throat plate is formed with a large counterbored aperture 50 beneaththe path of reciprocation of the sewing machine needle 14 and a hardenedinsert 51 with a flange 52 is secured to the throat plate by screws 53in a position filling the aperture 50. Preferably, as shown in FIG. 4,the lower extremity of the hardened insert 51 is formed with a slightlytapered face 55 onto which opens a needle aperture 56 which is formedthrough the hardened insert the juncture of the needle aperture 56 withthe underface 55 of the insert is formed at one side with asubstantially straight portion 57 extending substantially radially ofthe stud shaft 23 so as to define a stationary cutting edge or ledgerblade for cooperation with the cutting edge 42 of the blade 41. Theincline of the face 55 serves to flex the cutting blade 41 slightly asit passes beneath the hardened insert which insures close cooperationtherebetween. As illustrated in FIGS. 2 and 4, a notch 60 is provided inthe underface 55 of the insert leading from the needle aperture 56 on anangle radially inwardly. Any thread which is directed from the needleaperture radially outward will pass over the sharpened ledger blade edge57 and will be cut when the cutting edge 42 of the knife passes. Anythread which is drawn radially inward beneath the needle aperture 56will be accommodated in the notch 60 and will not be severed whenthe'knife cutting edge passes. The tapered converging fingers 35 and 36and the arched arm 38 of the thread manipulating plate serve selectivelyto urge the bobbin thread and each limb of a loop of needle threadrelatively to the needle aperture 55 as will now be described so thatthe bobbin thread will be severed and only the work limb of the needlethread loop but not the supply limb thereof will be cut when the cuttingedge 42 of the knife blade passes across the hardened insert 51.

In solid lines in FIG. 2 the parts of the thread trimming mechanism ofthis invention are illustrated in an at-rest position which the partsoccupy during normal sewing operations of the sewing machine. ln dottedlines in FIG. 2 is illustrated an intermediate position of the threadmanipulating plate 30 preferably derived as a result of the operation ofa conventional cam in the sewing machine. In this intermediate positionthe arched arm 38 will have traversed the needle aperture 56 preferablyat a time when no needle thread extends from the needle aperture.Consequently, the bobbin thread will be urged radially outwardly by thearched arm 38 positioning thebobbin thread away from the notch 60 sothat the bobbin thread will be cut by the knife blade 41 aftersufficient bobbin thread has been pulled off to insure a sufficientlylong starting end for the next stitch group. ln solid lines in FIG. 3 isillustrated a second intermediate position of the thread manipulatingplate 30, also preferably the result of the operation ofa conventionalcam on the sewing machine in which the tapered converging fingers 35 and36 are shifted into a loop of needle thread being manipulated by theloop taker 12 of the sewing machine. This position of parts is alsoillustrated with the threads in FlG. 6. The limb of the needle threadloop N" which extends to the work will be deflected radially outwardlyby the finger 36 so that this limb will be cut; while the limb of needlethread loop N which extends to the needle and to the thread supply willbe deflected radially inwardly by the finger 35 so that this thread limbwill enter the notch 60 and will not be severed by the knife blade 41.The bobbin thread B will be deflected radially outwardly by the archedarm 38 so that the bobbin thread will be cut. As shown in dotted linesin FIG. 3, the cutting position of the trimming mechanism of thisinvention, which is preferably attained by a manual operation of anyconventional operator influenced device on the sewing machine causes thecutting edge 42 of the knife blade to traverse the ledger blade 57 andsever the bobbin thread B and the work limb N' of the needle threadloop.

Since the thickness of the hardened insert 51 provides the onlylimitation on the length of thread ends which will extend from the workafter the threads have been severed by the device of this invention, itwill be 7 apparent that by using this invention the length of thesethread ends may be reduced to a minimum without interference with stitchformation.

Having set forth the nature of this invention, what is claimed hereinis:

1. An underbed thread trimming device for simultaneously severing thebobbin thread arid one limb of the needle thread loop on a lock-stitchsewing machine and in which both a movable thread cutting blade and acooperating stationary ledger blade are carried on the sewing machinework-supporting throat plate, characterized in that the movable threadcutting blade is formed with a sharpened edge arranged beneath andsubstantially parallel to the throat plate for cooperation with asimilarly oriented stationary ledger blade provided along the bottomface of an insert secured in the throat plate through which insert aneedle hole is formed, and in which the cooperative cutting engagementof said sharpened movable blade with said stationary ledger blade isinterrupted at one side of said needle hole by a thread clearance notchformed in one of said blades, and a thread manipulating plate is carriedwith said movable blade and formed with thread deflecting fingers forpositioning limbs of thread from said needle hole relatively to saidthread clearance notch.

1. An underbed thread trimming device for simultaneously severing thebobbin thread and one Limb of the needle thread loop on a lock-stitchsewing machine and in which both a movable thread cutting blade and acooperating stationary ledger blade are carried on the sewing machinework-supporting throat plate, characterized in that the movable threadcutting blade is formed with a sharpened edge arranged beneath andsubstantially parallel to the throat plate for cooperation with asimilarly oriented stationary ledger blade provided along the bottomface of an insert secured in the throat plate through which insert aneedle hole is formed, and in which the cooperative cutting engagementof said sharpened movable blade with said stationary ledger blade isinterrupted at one side of said needle hole by a thread clearance notchformed in one of said blades, and a thread manipulating plate is carriedwith said movable blade and formed with thread deflecting fingers forpositioning limbs of thread from said needle hole relatively to saidthread clearance notch.